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Publication Details
AFRICAN RESEARCH NEXUS
SHINING A SPOTLIGHT ON AFRICAN RESEARCH
chemistry
Densification and depression in glass transition temperature in polystyrene thin films
Langmuir, Volume 30, No. 39, Year 2014
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Description
Ellipsometry and X-ray reflectivity were used to characterize the mass density and the glass transition temperature of supported polystyrene (PS) thin films as a function of their thickness. By measuring the critical wave vector (qc) on the plateau of total external reflection, we evidence that PS films get denser in a confined state when the film thickness is below 50 nm. Refractive indices (n) and electron density profiles measurements confirm this statement. The density of a 6 nm (0.4 gyration radius, Rg) thick film is 30% greater than that of a 150 nm (10Rg) film. A depression of 25 °C in glass transition temperature (Tg) was revealed as the film thickness is reduced. In the context of the free volume theory, this result seems to be in apparent contradiction with the fact that thinner films are denser. However, as the thermal expansion of thinner films is found to be greater than the one of thicker films, the increase in free volume is larger for thin films when temperature is raised. Therefore, the free volume reaches a critical value at a lower Tg for thinner films. This critical value corresponds to the onset of large cooperative movements of polymer chains. The link between the densification of ultrathin films and the drop in their Tg is thus reconciled. We finally show that at their respective Tg(h) all films exhibit a critical mass density of about 1.05 g/cm3 whatever their thickness. The thickness dependent thermal expansion related to the free volume is consequently a key factor to understand the drop in the Tg of ultrathin films. © 2014 American Chemical Society.
Authors & Co-Authors
Vignaud, Guillaume
France, Lorient
Institut de Recherche Dupuy de Lôme Irdl
Chébil, Mohamed Souheib
France, Le Mans
Institut Des Molécules et Matériaux du Mans
Delorme, Nicolas
France, Le Mans
Institut Des Molécules et Matériaux du Mans
Beuvier, Thomas
France, Le Mans
Institut Des Molécules et Matériaux du Mans
Grohens, Yves
France, Lorient
Institut de Recherche Dupuy de Lôme Irdl
Gibaud, Alain
France, Le Mans
Institut Des Molécules et Matériaux du Mans
Statistics
Citations: 79
Authors: 6
Affiliations: 3
Identifiers
Doi:
10.1021/la501639z
ISSN:
07437463
Research Areas
Mental Health